1. Field of the Invention
The present invention relates to an image forming method, and an image forming apparatus, in which an electrostatic latent image formed in an image forming process such as electrophotography, electrostatic printing and electrostatic recording is developed with a magnetic toner.
2. Related Background Art
As a developing method making use of a one-component magnetic toner, the developing method using a conductive magnetic toner as disclosed in U.S. Pat. No. 3,909,258, etc. is known in the art.
In such a developing method, however, toners are required to be substantially conductive, and, in the case of conductive toners, it has been difficult to transfer a toner image formed on a latent image carrying member to a final image supporting member, e.g., plain paper, by utilizing an electric field.
A proposal has been made on a novel developing method that can solve problems involved in developing methods making use of conventional one-component magnetic toners (for example, Japanese Patent Applications Laid-open No. 55-18656 and No. 55-18659). According to this method, an insulating magnetic toner is uniformly applied to a cylindrical toner carrying member having a magnet in its inner part, and the toner carrying member is opposed to a latent image carrying member without being in contact therewith, in the state of which development is carried out. As a method of forming a magnetic toner layer on the toner carrying member, there is a method in which a coating blade is used, provided at an outlet of a toner container. For example, the image forming apparatus illustrated in FIG. 1 comprises a toner carrying member 2, and a blade 1a comprised of a magnetic material, provided at the position opposed to a magnetic pole N1 of a fixed magnet built in the toner carrying member 2. A magnetic toner is caused to rise along the magnetic line of force between the magnetic pole and the magnetic material blade, and the toner having risen is cut with an edge at the tip of the blade so that the thickness of the magnetic toner layer can be controlled by utilizing the action of the magnetic force (see, for example, Japanese Patent Application Laid-open No. 54-43037).
At the time of development, a low-frequency alternating voltage is applied between the toner carrying member and a conducting base body of the latent image carrying member so that the magnetic toner may reciprocate between the toner carrying member and the latent image carrying member. Desired good development can be thus carried out. In this developing method, the magnetic toner has insulation properties and hence can be readily electrostatically transferred.
The image forming apparatus illustrated in FIG. 2 is equipped with a developing device 7 holding therein a toner 10, and a latent image carrying member 9 such as a photosensitive drum used in electrophotography or an insulating drum used in electrostatic recording (hereinafter "photosensitive member" or "photosensitive drum").
In the developing method making use of such apparatus, very important are the subject (A): to uniformly apply the magnetic toner to the toner carrying member and the subject (B): to prevent or decrease contamination on the surface of the toner carrying member, caused by some constituents in the magnetic toner. The subject (A) and the subject (B), however, have the relation that they conflict with each other, and it is difficult to settle both of them at the same time.
In the subject (A), as a method of uniformly applying the magnetic toner to the toner carrying member, a proposal is made on a developing device capable of forming a toner coat layer on a toner carrying member in a stable state for a period long enough from a practical viewpoint (Japanese Patent Application Laid-open No. 57-66455). This developing device is a superior developing device, which is comprised of a toner carrying member made to have a rough surface with a specific irregularity by sand-blasting using amorphous particles and thus can maintain on the surface of the toner carrying member the state of a toner coat that is uniform, free from uneveness and always good for a long period of time. As shown in FIG. 10, a cylindrical toner carrying member has an embodiment in which its surface is provided over the entire surface with numberless cuts or protuberances formed in random directions.
In the developing device comprised of a toner carrying member having such a specific surface state, some of magnetic toners to be applied tend to cause adhesion of a toner or constituents in a toner to the surface of the toner carrying member. Hence the surface of the toner carrying member may be contaminated, consequently tending to cause a lowering of image density at the initial stage and, when it has become more contaminated as a result of copying on a large number of sheets, cause image blank areas to occur in every period of rotation of the toner carrying member. This is due to the fact that the constituents in a toner adhere to the slopes of projections and the hollows or concavities, of the surface of the toner carrying member to cause poor static charge of magnetic toner particles, resulting in a lowering of the quantity of static charge in a magnetic toner layer.
In general, the constituents of a magnetic toner are formed of materials such as a binder resin, a magnetic material, a charge control agent and a release agent. Materials are selected so that the contamination on the surface of the toner carrying member can be prevented. Hence, under present circumstances, the materials are limited.
In the subject (B), as a method of preventing or decreasing contamination on the surface of the toner carrying member, it has been clear that it is advantageous to make smoother the surface of a toner carrying member. Such a method, however, is experimentally found to tend to bring about a non-uniform toner coat when a magnetic toner has a volume average particle diameter of 12 .mu.m or more, to cause uneveness in toner images, often resulting in no formation of good toner image. The phenomenon in which this non-uniform toner coat is formed was observed in detail by carrying out running of a developing device without image-copying to reveal the following.
At the initial stage of the running without image-copying, the toner coat layer becomes excessively thick when the surface of the toner carrying member is smooth, although its cause is unclear. When the toner thickness is controlled using the blade 1a, the toner gradually comes to bulge out on the photosensitive member 9 side of the blade 1a (the part A in FIG. 2). As shown in FIG. 3 as a partially enlarge cross section, the toner forms a heap 10a at the part A. When this toner heap reaches a certain limit quantity, it moves to the surface of a sleeve 2 on the outside of the toner container because of the transporting force of the sleeve 2 to give coat uneveness 3a. When the coat uneveness 3a (a toner uneveness 3a. When the coat uneveness 38 (a toner mass) is present on the toner layer 3 formed in a uniform coating, this uneveness comes out as an uneveness on a toner image. The latter uneveness corresponds to density uneveness and fog. The toner coat uneveness 3a is found to have the shape of a rectangular spot, a wavelike spot, a wavelike pattern, or the like.
As discussed above, it has been very difficult for the conventional developing method to settle both the subject (A) and the subject (B) at the same time. This tendency becomes more remarkable under conditions of a low humidity or in a developing device designed to have a toner carrying member that rotates at a higher peripheral speed.
With an intent to improve image quality, European Patent Application Publication No. 0314459 proposes a magnetic toner having a volume average particle diameter of from 4 .mu.m to 9 .mu.m and also having a specific particle size distribution, and an image forming apparatus making use of such a magnetic toner.
European Patent Application Publication No. 0331425 also proposes an image forming method in which a magnetic toner having a volume average particle diameter of from 4 .mu.m to 9 .mu.m and also having a specific particle size distribution is fed to a toner carrying member having the surface with an uneveness comprising sphere-traced concavities and thus an electrostatic latent image is developed.
It, however, has been sought to provide a more improved image forming method or apparatus that has been adapted to the developing speed made increasingly higher.